In many telecommunication systems, communications networks are used to exchange messages among several interacting spatially-separated devices. Networks may be classified according to geographic scope, which could be, for example, a metropolitan area, a local area, or a personal area. Such networks may be designated respectively as a wide area network (WAN), metropolitan area network (MAN), local area network (LAN), or personal area network (PAN). Networks also differ according to the switching/routing technique used to interconnect the various network nodes and devices (e.g., circuit switching vs. packet switching), the type of physical media employed for transmission (e.g., wired vs. wireless), and the set of communication protocols used (e.g., Internet protocol suite, SONET (Synchronous Optical Networking), Ethernet, etc.).
Wireless networks are often preferred when the network elements are mobile and thus have dynamic connectivity needs, or if the network architecture is formed in an ad hoc, rather than fixed, topology. Wireless networks employ intangible physical media in an unguided propagation mode using electromagnetic waves in the radio, microwave, infrared, optical, etc. frequency bands. Wireless networks advantageously facilitate user mobility and rapid field deployment when compared to fixed wired networks.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-input-single-output (SISO), multiple-input-single-output (MISO), single-input-multiple-output (SIMO), or a multiple-input-multiple output (MIMO) system.
Additionally, in wireless communications systems, spectrum bandwidth and base station transmit power may be regulated. To design around such constraints, multiple-input multiple-output (MIMO) systems may provide an increased peak data rate, spectral efficiency, and quality of service. A MIMO system consists of transmitter(s) and receiver(s) equipped, respectively, with multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A variant of a MIMO system that still presents gains compared to single-input single-output (SISO) systems is a single-input multiple-output (SIMO) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NV independent channels, which are also referred to as spatial eigenchannels, where 1≦NV≦min {NT, NR}.
MIMO systems can provide improved performance (e.g. higher throughput, greater capacity, or improved reliability, or any combination thereof) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. It should be appreciated that although SIMO systems afford a somewhat lesser improvement in performance, such systems avoid complexity at the receiver, by employing only a single antenna in the user equipment and relying on multiple antennas at base stations.
The devices in a wireless network may transmit/receive information between each other via wireless signals. Communication systems experience channel fading or other interference. This interference may make it more difficult to decode received signals. Devices may have a need for preventing interference between wireless signals transmitted at different frequencies to reduce interference within the system and increase the bandwidth over which signals may be transmitted.
One method to mitigate challenges associated with channel interference is with the use of beamforming. With beamforming, each transmit antenna of a transmitter is not operated independently to form a spatial stream. Instead, the transmit antennas transmit a linear combination of spatial streams, with the combination chosen so as to optimize the response at an intended receiver.
Smart antennas are arrays of antenna elements, each of which transmit a signal with a predetermined phase offset and relative gain. The net effect of the array is to direct a transmission beam in a predetermined direction. The beam is steered by controlling the phase and gain relationships of the signals that excite the elements of the array. Thus, smart antennas direct a beam to each individual mobile unit (or multiple mobile units) as opposed to radiating energy to all mobile units within a predetermined coverage area as conventional antennas typically do. Smart antennas increase system capacity by decreasing the width of the beam directed at each mobile unit and thereby decreasing interference between mobile units. Such reductions in interference result in increases in signal to interference and signal to noise ratios that improve performance and/or capacity. In power controlled systems, directing narrow beam signals at each mobile unit also results in a reduction in the transmit power required to provide a given level of performance.
While utilization of beamforming may improve performance in some dimensions, the implementation of beamforming has associated costs. For example, beamforming of a plurality of transmit antennas may rely on statistical information generated by a receiver. Generation of the beamforming information, or a “beamforming report,” may impose burdens on the receiver in terms of computational processing requirements and power consumption. Therefore, improved methods and apparatus for performing beamforming are desired.